1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) apparatus and a static magnetic field correction method and more specifically to a technique to adjust the nonuniformity of a static magnetic field which is caused by a patient.
2. Description of the Related Art
With conventional MRI apparatus, to adjust the uniformity of a static magnetic field which varies from patient to patient, a technique called shimming is carried out. This technique usually involves collecting data reflecting the influence of a static magnetic field, then calculating the amount of correction of the static magnetic field on the basis of that data, determining a value of current corresponding to the amount of correction, and supplying a current of that value to a correction coil.
Typically, the collection of data reflecting the influence of a static magnetic field is carried out on a region of a sufficient size to accommodate a patient. In that case, however, data from regions which are not intended for diagnostic purposes will be contained. If the distribution of strength of a static magnetic field within such regions greatly varies from that within a region to be diagnosed (an imaging region), the proper amount of correction cannot be calculated in some cases. For instance, the heart portion and the breast wall portion greatly vary in the static magnetic field conditions in many cases. When data from these portions are mixed, therefore, errors will arise in the calculated amount of correction, which may result in a failure to make the optimum magnetic field adjustment.
To solve such a problem, a method has been proposed which calculates the amount of correction on the basis of data collected from only the inside of a region of interest (ROI) which is set limited to the surrounding of a region for which an imaging diagnosis is to be made. This method is called local shimming. However, in order to correctly collect data concerning an imaging region, it is required to set a region including the surroundings of the imaging region as the ROI. That is, the ROI has to be made sufficiently larger than the imaging region. For this reason, data from the ROI will contain data from other regions than the imaging region, which may result in a failure to calculate the proper amount of correction.
A technique to adjust a static magnetic field according to the location or size of an imaging region has also been proposed (see, for example, Japanese Patent No. 3237964).
With the conventional MRI apparatus, as described above, difficulties are involved in calculating the proper amount of correction and consequently the static magnetic field within an imaging region cannot be adjusted properly.